Field
The disclosed concept relates generally to electrical switching apparatus and, more particularly, to an electrical switching apparatus such as a circuit breaker. The disclosed concept also relates to clinch joint assemblies for circuit breakers.
Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include an operating mechanism which opens electrical contact assemblies to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions. The operating mechanism is designed to rapidly open and close separable contacts. The operating mechanism is structured to be latched and thereby maintain the contacts in a closed configuration. A trip unit is structured to detect over-current conditions. When an over-current condition is detected, the trip unit releases the operating mechanism latch thereby allowing biasing elements to bias the operating mechanism and contacts, to an open configuration. Generally, a circuit breaker is assigned a size and a “withstand” value. The size of the circuit breaker is substantially related to the size of the circuit breaker housing assembly or frame. The circuit breaker withstand value involves a balance between blow-off forces generated by electric currents flowing in the breaker and contact forces generated on the movable conductor by the operating mechanism.
Many low-voltage circuit breakers, employ a molded housing having two parts, a first half or front part (e.g., a molded cover), and a second half or rear part (e.g., a molded base). The operating mechanism for such circuit breakers is often mounted to the front part of the housing, and typically includes an operating handle and/or button(s) which, at one end, is (are) accessible from the exterior of the molded housing and, at the other end, is (are) coupled to a pivotable pole shaft. Electrical contact assemblies, which are also disposed within the molded housing, generally comprise a conductor assembly including a movable contact assembly having a plurality of movable contacts, and a stationary contact assembly having a plurality of corresponding stationary contacts. The movable contact assembly is electrically connected to a generally rigid conductor of the conductor assembly by flexible conductors, commonly referred to as shunts. The movable contact assembly includes a plurality of movable contact arms or fingers, each carrying one of the movable contacts and being pivotably coupled to a contact arm carrier. The contact arm carrier is pivoted by a protrusion or arm on the pole shaft of the circuit breaker operating mechanism to move the movable contacts between an open, first position (not shown), wherein the movable contacts are not coupled to, and are not in electrical communication with, the corresponding stationary contacts, and a closed, second position (contact arm 58D, described below, is shown in the second position in FIG. 1), wherein the movable contacts are coupled to, and are in electrical communication with, the corresponding stationary contacts. The contact arm carrier includes a contact spring assembly structured to bias the fingers of the movable contact assembly against the stationary contacts of the stationary contact assembly in order to provide and maintain contact pressure when the circuit breaker is closed, and to accommodate wear.
The shunts typically comprise either copper wire ropes or layered copper ribbons, and are solidified at their ends using heat and pressure and then brazed to the rigid conductor at one end, and to the movable contact assembly contact arms at the opposite end. One of the disadvantages associated with known wire rope or braided-type shunts is that they do not fit well within the limited spacing which is available between the adjacent contact arms of the movable contact assembly. Specifically, the body of such shunts tends to expand outward and occupy more than the width of the finger, thus interfering with adjacent structures. The wire ropes also tend to bunch together during short circuit events, thus inhibiting the flexibility of the assembly. This is problematic in view of the compound motion which the fingers experience as a result of the well-known “heel-toe” and/or “blow-on” arcing schemes which are commonly employed by low-voltage circuit breakers. See, e.g., U.S. Pat. No. 6,005,206.
To accommodate the movement of the contact finger during separation from a stationary contact, an elongated shunt is typically disposed in an “S” shape for use, i.e., a “use shape.” That is, as used herein a “use shape” is the overall shape of the shunt, as opposed to, for example, the cross-sectional shape, of a shunt prior to an over current event. This may also be identified as the “resting shape.” In an electrical switching apparatus having a greater withstand value, e.g., a circuit breaker structured for a higher voltage, elongated shunts create magnetic fields during an overcurrent event. Such magnetic fields from adjacent shunts, as well as the movement caused by the operating mechanism, cause the shunt to rapidly change shape in an extreme compound deflection, or colloquially, an extreme “wiggle,” during an over current event. This motion causes the shunt to wear and creates uncontrollable forces that affect the carrier and contact arms.
Layered ribbon-type shunts also suffer from a number of unique disadvantages. Among them is the fact that they are typically V-shaped, thus having a single relatively sharp bend which undesirably creates an area of stress concentration. This V shape also consumes a substantial amount of valuable space within the molded housing of the circuit breaker.
Thus, there is a problem with the size and configuration, including the use shape, of shunts. That is, shunt loads are not isolated from the movable contact assembly contact arms, and, longer shunts are subject to extreme compound deflection.
Further, when a current is passing through the shunts, the shunts have a magnetic field that produces forces that act upon other elements of the electrical contact assemblies. These magnetic fields and corresponding forces are variable due to the variable configuration of the shunts, i.e., when the wire ropes also tend to bunch together during short circuit events. This is a disadvantage as the variable forces enhance, or detract from, the opening forces created by the operating mechanism. That is, having an operating mechanism that has variable opening characteristics is a disadvantage.
One improvement relating to electrical contact assemblies is the use of a clinch joint assembly. A clinch joint assembly eliminates the shunts by including a slotted conductor having a bifurcated member, such as a yoke, supporting an axle member. The movable contact assembly contact arm is rotatably disposed on the axle. The yoke is laterally biased against the movable contact assembly contact arm, i.e., the yoke holds the movable contact assembly contact arm tightly or “clinches” the movable contact assembly contact arm. The lateral bias creates a torque on the movable contact assembly contact arm that resists rotation. The slotted conductor is coupled to the conductor assembly. Thus, electricity flows through the conductor assembly, the slotted conductor, and the movable contact assembly contact arm before reaching the movable contact. See, e.g., U.S. Pat. No. 4,245,203. In this configuration, the rotation of the contact arm is influenced, in part, by the lateral pressure or torque applied to the contact arm by the slotted conductor. It is noted that, in this configuration, the lateral bias torque is created by friction. As the friction is affected by the contacting surface area on the yoke and the movable contact assembly contact arms, manufacturing tolerances and other factors affect the torque. That is, the level of torque balance control could be improved.
In this configuration, the movable contact assembly is limited to a maximum of two contact arms. That is, the lateral bias applied by the yoke must apply bias in a controlled manner to the movable contact assembly contact arms so as to control the blow open characteristics of each arm. This is only possible with a two-arm configuration because the torqued applied by a yoke to a medial contact arm, i.e., a contact arm between two other contact arms, cannot be controlled. That is, because the fingers typically have the same geometry, i.e., same shape, and rotate about the same axle, the contact area between the adjacent surface of each finger could be large or small. That is, the “contact area” is variable due to the roughness/smoothness of each surface resulting in a different number of contact points over each surface, warping of the contact fingers, and other factors that affect the total area in actual contact on each contact finger lateral surface. This variable contact surface area creates a difference in the surfaces' coefficient of friction and variations in the coefficient of friction over a single contact finger lateral surface. Thus, when the contact fingers are compressed laterally, each finger is subject to a variable torque due to the differences in friction. In a two-finger configuration, each finger is subjected to friction created by the yoke, which due to the smaller contact area is negligible relative to the larger lateral surface contact area, and the lateral surface contact area. When there are two contact fingers, the friction acting on the lateral surface contact area is the same because it is the same lateral surface contact area. That is, by definition, the lateral surface contact area of a first contact arm disposed against a second contact arm is the same as the lateral surface contact area of that second contact arm disposed against that first contact arm.
This is not true of a stack of three or more contact arms. By way of an analogy, imagine assembling three or more paper plates in a stack with a central axle through the stack. Depending on how they are assembled, the flatness, or non-flatness, creates more or less friction between adjacent plates. If a rotational force was applied equally to each plate, the plates would spin at different rates due to the differences in friction between adjacent plates. This is true of contact arms as well.
This is a disadvantage because the rating, i.e., withstand value, or, the size, of the circuit breaker is limited by the size of the movable contact assembly contact arms. That is, for a higher rating, the size of the movable contact assembly contact arms, and therefore the size of the circuit breaker, must be increased.
Thus, there is a problem with the size and configuration of clinch joint assemblies. As noted above, the level of torque balance control could be improved while accommodating manufacturing tolerances. Further, the limited number of movable contact assembly contact arms allowed by present clinch joint assemblies is a problem.
An electrical switching apparatus with a higher withstand value may include elements of both a movable contact assembly and a clinch joint assembly. That is, an air circuit breaker is structured to withstand greater currents and thereby allow downstream circuit breakers to open during a relatively less intense over-current event. Thus, by way of example, a single room in a hospital may have its power interrupted, rather than the entire wing of the hospital. During a relatively more intense over-current event, the air circuit breaker will open. Moreover, during such an over-current event, it is better for the air circuit breaker to open as quickly as possible. This is accomplished by having a number of fingers on an air circuit breaker clinch joint assembly “blow open,” i.e., pivot quickly, in response to a magnetic field generated by the over current condition. Further, in response to a trip unit detecting the same over current condition, the air circuit breaker operating mechanism will be actuated and move the entire air circuit breaker clinch joint assembly away from the stationary contacts. Thus, the movable contact assembly contact arms “blow open” first, then the entire clinch joint assembly is moved away from the stationary contacts. Because the clinch joint assembly is not fixed to the conductor, the movable contact assembly included shunts to couple, and provide electrical communication between, the conductor and the clinch joint assembly. In view of the higher voltage for which an air circuit breaker is rated, the amount of “wiggle” a shunt experiences during an over current condition is increased. That is, an air circuit breaker that utilizes a moving clinch joint assembly is subject to the problems of both clinch joint assemblies and shunts noted above.
There is a need, therefore, for elements of the movable contact assembly (e.g., shunts) which solve the problems noted above. There is a further need for elements of the movable contact assembly (e.g., a clinch joint assembly) which solve the problems noted above. Accordingly, there is room for improvement of conductor assemblies for electrical switching apparatus such as, for example, air circuit breakers.